Hiding a Dagger Behind a Smile

In the Tang Dynasty, there was a minister called Lu Yifu who was always affable and smiling. But in his heart he was very sinister and ruthless. He constantly schemed against people he saw as possible rivals. He was called 'The knife in the smile'. 

This idiom, derived from the above story, means disguising a ruthless nature behind a pleasant appearance.

唐朝有一个大臣李义府，表面上对人温和恭顺，跟人谈话总是面带微笑。但是，内心深处却十分阴险毒辣。凡是能力比他强大的人，他都要设法加以打击和谋害。当时人们称他是“笑中刀”。 

“笑里藏刀”是由“笑中刀”转化而来的，比喻表面装得善良和气，而内心暗藏阴险毒辣。